The Big Picture
Page 34
09
description of reality.
10
What’s new is that Newton and Laplace, even if they had thought of
11
their ideas as only accurate in a certain regime, had no way of knowing how
12
far that regime extended. Newtonian gravity works very well for the Earth
13
or Venus; it eventually starts breaking down when we consider the orbit of
14
Mercury, whose tiny precession became some of the strongest evidence in
15
favor of Einstein’s general relativity. But Newton would have had no idea
16
how far his theory might be accurate.
17
With effective field theory, however, that’s exactly what we have. An
18
effective field theory describes everything that happens to a certain set of
19
fields, as long as the energies are lower than a certain cutoff, and distances
20
are larger than a certain lower limit (as set by experiment). Once we have
21
the parameters of the effective theory pinned down, we know what will
22
happen to our fields in any experiment we can imagine within its domain
23
of applicability, even if we haven’t done that experiment yet.
24
It’s this special feature of quantum field theory that gives us the confi-
25
dence to make such audacious claims about the scope of our knowledge.
26
27
•
28
There are a million ways to misinterpret “The laws of physics underlying
29
everyday life are completely known.” While it’s an undeniably bold claim,
30
it would be easy to mistake it for something even more grandiose than it
31
actually is, and then dismiss that exaggerated claim. It certainly does not
32
imply that we know all of physics.
33
Nor does it, by any wild stretch of the imagination, imply that we know
34
how everything works at the level of the everyday. Nobody in their right S35
mind thinks that we have, or are close to having, complete theories of
N36
191
Big Picture - UK final proofs.indd 191
20/07/2016 10:02:46
T H E B IG PIC T U R E
01
biology or neuroscience or the weather, or for that matter of the flow of
02
electricity through ordinary materials. Those phenomena need to be com-
03
patible with the Core Theory, but the phenomena themselves are emergent.
04
As we discussed in chapter 12, understanding emergent phenomena is a
05
matter of discovering new knowledge— finding those patterns (where they
06
exist) that allow us to describe simple behaviors out of many underlying
07
moving parts. Sometimes the simple demand of compatibility with an un-
08
derlying theory tells us a great deal, as in the case of planets moving around
09
the sun. Conservation of momentum immediately tells us that the Earth
10
won’t go careening off in a random direction; the absence of long- range
11
forces other than gravity and electromagnetism tells us that you can’t bend
12
spoons with your mind. But for the most part, there is a wide gap between
13
knowing a theory at one level and knowing the emergent theories that are
14
related to it by coarse- graining.
15
The success of the Core Theory, and our understanding of its domain of
16
applicability, thanks to the principles of effective field theory, implies that
17
there is an enormous presumption (a high Bayesian credence) in favor of
18
understanding macroscopic phenomena in terms that are compatible with
19
the underlying laws of physics. There can always be exceptions. But as David
20
Hume would have said, if you believe that any one particular case is a true
21
example of the Core Theory being violated, your evidence in favor of it
22
needs to be strong enough to overcome the enormous amounts of evidence
23
to the contrary.
24
•
25
26
Even accepting that science never proves anything and that surprises are
27
always possible, there are still some small loopholes in our arguments that
28
the laws of physics underlying everyday life are completely known. It would
29
be intellectually dishonest not to acknowledge them, so here we go.
30
The most straightforward loophole would be if quantum field theory
31
were just flat- out wrong in the domain that includes everyday life. For ex-
32
ample, if there were physical effects that stretched from one particle to an-
33
other, but not via anything like a quantum field. This seems very unlikely,
34
on general grounds; once you accept the basic principles of relativity and
35S
quantum mechanics, you are more or less forced into accepting quantum
36N
field theory. In regions where gravity is strong, like the Big Bang and black
192
Big Picture - UK final proofs.indd 192
20/07/2016 10:02:46
t h E E F F E C t I v E t h E OR y OF t h E E v E R y dA y WO R l d holes, field theory may very well break down. There aren’t any black holes
01
in your living room, happily. But for the sake of completeness, we should
02
admit that it’s always a possibility.
03
The second possible loophole, arguably more plausible than the first, is
04
the looming problem that we don’t fully understand quantum mechanics.
05
It’s possible that we have in hand all of the basic pieces of quantum ontology 06
(wave functions, the Schrödinger evolution equation), and the founda-
07
tional work that remains is to interpret how that formalism describes the
08
real world. In that case, this loophole closes with a slam. Indeed, in all of
09
the most popular approaches to quantum mechanics, there really isn’t any
10
loophole here at all; there’s no place in quantum dynamics for the general
11
principles of effective field theory to be violated.
12
But because we don’t all agree on the correct formulation of quantum me-
13
chanics, it’s conceivable that none of the most popular alternatives is correct.
14
We can imagine that the correct theory of quantum mechanics will ultimately
15
tell us that wave functions don’t really collapse randomly, for example; perhaps
16
there are subtle features of quantum measurement that have thus far eluded
17
experimental detection, but wil
l end up playing an important role in how we
18
come to understand biology or consciousness. It’s possible.
19
Another loophole is the possibility that “new physics” lurks not in new
20
dynamic laws but in something we don’t yet appreciate about the initial
21
conditions of the universe. A kind of prearrangement, rather than predes-
22
tination. The early universe seems to have been a very simple, low- entropy
23
place, which means (following Boltzmann’s definition of entropy) there
24
aren’t many states it could have been in. But it’s at least conceivable that it
25
was in a very special state featuring extremely subtle correlations that work
26
to influence our world today. We have no direct reason to believe that’s
27
true, but it deserves a place on our list of loopholes.
28
Finally, there is the manifest loophole that describing the world in terms
29
of physics alone might not be good enough. There might be more to reality
30
than the physical world. We’ll leave serious discussion of that possibility for
31
chapter 41.
32
The most likely scenario for future progress is that the Core Theory
33
continues to serve as an extremely good model in its domain of applicability
34
while we push forward to understand the world better at the levels above,
S35
below, and to the side. We used to think that atoms consisted of a nucleus
N36
193
Big Picture - UK final proofs.indd 193
20/07/2016 10:02:46
T H E B IG PIC T U R E
01
and some electrons orbiting around it; now we know that the nucleus is
02
made of protons and neutrons, which are in turn made of quarks and glu-
03
ons. But we didn’t stop believing in nuclei when we learned about protons
04
and neutrons, and we didn’t stop believing in protons and neutrons when
05
we learned about quarks and gluons. Likewise, even after another hundred
06
or thousand years of scientific progress, we will still believe in the Core
07
Theory, with its fields and their interactions. Hopefully by then we’ll be in
08
possession of an even deeper level of understanding, but the Core Theory
09
will never go away. That’s the power of effective theories.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35S
36N
194
Big Picture - UK final proofs.indd 194
20/07/2016 10:02:46
01
02
25
03
04
Why Does the Universe Exist?
05
06
07
08
09
10
11
12
I
13
fell in love with the universe at an early age. Lying in bed at night, ready
14
to fall asleep, I’d often be thinking about the expansion of space, and
15
what things were like back near the Big Bang, and what other kinds of
16
universes could exist— until I would come to the thought: What if our
17
universe hadn’t existed at all? What if there were simply nothing? That
18
would be it. No sleep for me that night.
19
These are classic questions, and behind them lurks a conviction that the
20
existence of the universe demands some kind of explanation. In a 1697 essay
21
entitled “On the Ultimate Origin of Things,” Gottfried Leibniz— whom
22
we remember as the proponent of the Principle of Sufficient Reason and the
23
Principle of the Best, as well as the coinventor of calculus— argued that we
24
should be somewhat surprised that anything exists at all. Nothingness, af-
25
ter all, is simpler than any one particular existing thing ever could be; there
26
is only one nothing, and many kinds of something. More recently, British
27
philosopher Derek Parfit has sympathized, saying that “it can seem aston-
28
ishing that anything exists.”
29
Just because these questions are common, it doesn’t mean they’re the
30
right ones to ask. Sidney Morgenbesser, a much- beloved professor of phi-
31
losophy at Columbia University, renowned for his aphoristic wisdom, was
32
once asked, “Why is there something, rather than nothing?”
33
“If there were nothing,” Morgenbesser immediately replied, “you’d still
34
be complaining.”
S35
N36
195
Big Picture - UK final proofs.indd 195
20/07/2016 10:02:46
T H E B IG PIC T U R E
01
Beyond the worries and the witticisms, there are two interesting ques-
02
tions facing us, similar- sounding but different in important ways.
03
04
1. Could the universe, possibly, simply exist? Can we at least
05
imagine reasonable scenarios in which the universe simply
06
is, all by itself, or is it necessary to imagine something out-
07
side the universe in order to account for its existence?
08
2. What is the best explanation for the existence of the uni-
09
verse? If we need to invoke something outside the universe
10
to account for its existence, what is that thing? And is it
11
better or simpler to not invoke anything additional at all?
12
13
Following Aristotle, the fact that the universe exists is often cited as
14
evidence in favor of the existence of God. The universe is specific and con-
15
tingent, the argument goes; it could easily have been otherwise. So there
16
must be something that explains the universe, and then something that
17
explains that thing, and so on through the chain of reasons. To avoid diving
18
down a rabbit hole of infinite regress, we need to invoke a necessary being—
19
one that must exist and could not have been otherwise, and therefore re-
20
quires no explanation. And that being is God.
21
Poetic naturalists don’t like to talk a
bout necessities when it comes to
22
the universe. They prefer to lay all the options on the table, then try to fig-
23
ure out what our credences should be in each of them. Maybe there is an
24
ultimate explanation; maybe there is an infinite chain of explanations;
25
maybe there is no final explanation at all. The progress of modern physics
26
and cosmology has sent a fairly unequivocal message: there’s nothing wrong
27
with the universe existing without any external help. Why it exists the par-
28
ticular way it does, rather than some other way, is worth exploring.
29
•
30
31
Let’s start with the relatively straightforward, science- oriented question:
32
could the universe exist all by itself, or does it need something to bring it
33
into existence?
34
As Galileo taught us, one of the foundational features of modern phys-
35S
ics is that objects can move, and tend to do so, without any need for an ex-
36N
ternal cause or mover. Roughly speaking, the same goes for the universe.
196
Big Picture - UK final proofs.indd 196
20/07/2016 10:02:46
W h y d O E S t h E u n I v E R S E E x I S t ?
The scientific question to ask isn’t “What caused the universe?” or “What
01
keeps the universe going?” All we want to know is “Is the existence of the
02
universe compatible with unbroken laws of nature, or do we need to look
03
beyond those laws in order to account for it?”
04
This question is complicated by the fact that we don’t know what the
05
ultimate laws of nature actually are. Consider an issue that is inextricably
06
tied to why the universe exists: has it existed forever, or did it come into
07
existence at some particular moment, presumably the Big Bang?
08
Nobody knows. If we were Pierre- Simon Laplace, who believed in the
09
classical physics of Newton and scoffed at the idea that God would ever
10
interfere in the workings of nature, the answer would be easy: the universe
11
exists forever. Space and time are fixed and absolute, and it doesn’t really
12
matter what happens to the stuff that is moving around inside space. Time
13
stretches from the infinite past to the infinite future. Of course you are al-
14
ways welcome to consider other theories, but in unmodified Newtonian
15
physics the universe has no beginning.
16
Then in 1915 along comes Einstein and his theory of general relativity.
17
Space and time are subsumed into a four- dimensional spacetime, and space-